Trials test new dredging techniques

Van Oord and the Somerset Rivers Authority are working in partnership to test new techniques which may provide cheaper and better ways of maintaining tidal rivers.

Three weeks of trials are being carried out on the River Parrett near Bridgwater to see whether ‘water injection dredging’ and ‘agitation dredging’ can effectively move silt downstream to Bridgwater Bay, thereby preserving the River Parrett’s capacity.

Cllr John Osman, Somerset Rivers Authority (SRA) chairman, said: “This is the first time that such trials have been done on the Parrett in conjunction with a long-term programme of detailed monitoring.

“Potentially, these new methods offer big benefits: done at the right times, in the right places, they could be much cheaper, more effective, and better for the environment, local residents and farmers.”

If the trials are successful this could pave the way for vital maintenance of the Somerset Levels and Moors to be carried out in a way which provides better value for money for Somerset taxpayers and reduces disruption for residents.

In traditional dredging, silt is scooped out of the riverbed using excavators, either from the riverbank or with cranes mounted on special barges. The silt is then taken away in trucks and either deposited on agricultural land or used to strengthen existing riverbanks.

Jasper Blaauw, from Van Oord, said the methods being trialled will have significantly less impact on local people and the natural environment.

He said: “Our boat has been sat here for over a week, and no-one has noticed it. There is no noise issue – people have been walking their dogs or riding horses without being disturbed.

“Water injection dredging has a much lower impact than six or seven trucks and a crane. This is the culmination of 25 years of research and development.”

The trials started on Monday 14 November, in conjunction with detailed monitoring before, during and after. Along the trials section of the River Parrett, there is 150% more silt than there was in April, and in places this increases to four times as much.

Representatives from the SRA and the Somerset Drainage Board Consortium, gathered on the banks of the river on 16 November to see Van Oord’s water injection dredging boat in action.

Surveying has created a very precise profile of the river, which can be updated in near real-time. As the vessel passes, it re-measures. A screen on board the boat shows the relative positions of bed and injection point, making it possible to see what effect work is having, and to raise and lower the injection bar, and vary the pressure and volume as required for maximum effectiveness.

River water is pumped through the injection bar towards the soft silts in the riverbed, which become ‘super-saturated’ and more separated.

Van Oord call this a ‘density current’ – effectively, a different material to the water around it which, on an outgoing tide, should be carried out into Bridgwater Bay, and then disperse naturally.

The other method being trialled, ‘agitation dredging’, uses an articulated arm with a cutting device which rotates, and a suction hose. As the cutting head moves towards its target area, mud and water are vacuumed up, and blown back out into the top of the river. The idea is that silt is given much more time to carry before it drops down again and settles.

The trials aim to get a better understanding of where silt goes and the best ways to keep it moving.

21 November 2016